Active and passive monopulse direction finding systems have been successfully designed for many years when the available space for the antenna has been on the order of a wavelength or larger. Two direction finding systems generally used are the four element monopulse and the dual mode monopulse systems. The present invention deals with the dual mode monopulse system.
The dual mode direction finding technique is typically implemented by means of a four arm dual mode spiral antenna. This technique forms a circularly polarized, circularly symmetric sum pattern when excited for mode one and a circularly polarized, circularly symmetric difference pattern when excited for mode two. Amplitude comparison of the modes one and two gives the angle .theta. from the z axis. Phase comparison of modes one and two gives the angle .phi. about the z axis.
At frequencies below 1 GHz, the dual mode spiral antenna design is severely limited because the reduced size antenna aperture results in a narrow bandwidth, sensitivity to polarization, and poor accuracy. Attempts to reduce the size of a four spiral element monopulse antenna encounter element to element coupling and coupling to the ground plane structure which indicate that this approach is not feasible for applications requiring size reduction substantially below a one wavelength diameter. The present invention deals with the problem of an antenna which can provide accurate, broadband, all polarization direction finding at VHF through UHF frequencies in space-limited applications such as missiles, drones, RPV, and small aircraft. Because of space limitations, the total aperture size is much less than a wavelength demanding use of reduced size or electrically small antennas, and creating a need for extreme phase and/or amplitude accuracies in order to achieve acceptable angle accuracies. The use of an electrically small aperture makes it especially difficult to achieve both acceptable gain and high phase and amplitude balance over broadbands. Also, the small aperture antenna couple strongly to the airframe, with a resulting deterioration of antenna patterns and of derived angle information.